This application claims priority to European Application No. 02258468.4 filed Dec. 9, 2002.
1. Field of the Invention
The present invention relates to the determination of parameters for use in lithographic projection apparatus.
2. Description of the Related Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to structures that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning means can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT Patent Application Nos. WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g., in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing,” Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens;” however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
In lithography there is a problem known as the optical proximity effect. This is caused by the inherent difference in diffraction pattern for isolated features as compared to dense features. Dense features may include nested patterns and closely spaced periodic features. The optical proximity effect leads to a difference in critical dimension (CD) when dense and more isolated lines are printed at the same time. The lines are different when printed even though they are identical on the mask.
The optical proximity effect also depends on the illumination setting used. Originally, so-called conventional illumination modes have been used which have a disc-like intensity distribution of the illumination radiation at the pupil of the projection lens. However, with the trend to imaging smaller features, off-axis illumination modes have become standard in order to improve the process window, i.e., exposure and/or focus latitude, for small features. However, the optical proximity effect can become worse for off-axis illumination modes, such as annular illumination.
One solution to this problem has been to apply optical proximity correction (OPC) by biasing the different features on the reticle. According to one form of biasing, the features are biased, for example, by making the more isolated lines on the reticle somewhat thicker so that, in the image on the substrate, they are printed with the same transverse dimension as the dense lines. In another form of biasing, an end correction is applied so that the lines, whether isolated or dense, are printed with the correct length. However, at smaller pitches and with off-axis illumination, the greater the CD varies as a function of pitch, and so more line biasing has to be applied and the biasing becomes more complicated. Another form of optical proximity correction (OPC) is to use so-called “assist features” also known as “scatter bars” on the reticle to alter the diffraction of, for example, isolated features, such that they are printed with the correct dimension. OPC is discussed, for example, in U.S. Pat. No. 5,821,014 and in SPIE Vol. 4000, pages 1015 to 1023, “Automatic parallel optical proximity correction and verification system,” Watanabe et al.
Techniques are also known for optimizing the spatial intensity distribution of the radiation source dependent on the pattern being imaged. According to one method the radiation source is divided into blocks and the system is modeled as being equivalent to a point source at each block which is either on or off. For each source point in turn the resulting intensity at selected points on the substrate is calculated. An optimization routine is then used to calculate the optimum source distribution comprising a plurality of illumination source blocks so as to minimize the difference between the calculated intensity at the substrate and the ideal intensity at the substrate for best printing of the pattern. Another technique is to calculate the difference between the actual intensity and ideal intensity for every block of the radiation source and place them in rank-order. The overall illumination intensity distribution is obtained by accepting the source blocks in rank order until the illumination reaches a threshold. Further details of these techniques can be obtained from U.S. Pat. No. 6,045,976, incorporated herein by reference.
As will be appreciated, advanced software algorithms and very complex mask making is required for OPC and similarly advanced software is required for source optimization. There has been a problem of satisfactorily combining OPC with simultaneously optimizing the illumination intensity distribution and providing an adequate process window with sufficient process latitude for a range of features or groups of features.
It is an aspect of embodiments of the present invention to alleviate, at least partially, the above problem. According to embodiments of the present invention there is provided a method for determining a projection beam source intensity distribution and optical proximity correction rules for a patterning device for use with a lithographic projection apparatus including: a radiation system for providing the projection beam of radiation, a support structure for supporting patterning device, the patterning device serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, selecting a plurality of features of the desired pattern to be imaged, notionally dividing the radiation in the radiation system into a plurality of source elements, for each source element: calculating the process window for each selected feature and determining the optical proximity correction rules that optimize the overlap of the calculated process windows, selecting those source elements for which the overlapping of the process windows and the optical proximity correction rules satisfy specified criteria, and outputting data on the selected source elements, which source elements define a source intensity distribution and optical proximity correction rules.
A further aspect of embodiments of the present invention provides a computer system comprising a data processor and data storage, the data processor being adapted to process data in accordance with an executable program stored in the data storage, wherein the executable program is adapted to execute the above method.
The invention also provides a computer program comprising program code for executing on a computer the above method, and a computer program product carrying the computer program.
Another aspect of the invention provides a method of manufacturing a device using a lithographic projection apparatus including: a radiation system for providing a projection beam of radiation, a support structure for supporting a patterning device, the patterning device serving to pattern the projection beam according to a desired pattern, a substrate table for holding a substrate, a projection system for projecting the patterned beam onto a target portion of the substrate, the method including, providing a substrate which is at least partially covered by a layer of energy-sensitive material, providing a pattern it is desired to create on the substrate, providing patterning means on the support structure, creating a source intensity distribution in the radiation system which corresponds substantially to the sum of the selected source elements output by the above method, defining the pattern of the patterning device according to the pattern it is desired to image on the substrate modified according to the optical proximity correction rules output by the above method, and exposing a target area of the layer of energy-sensitive material on the substrate, using the patterned radiation beam, within the process window output by the above method, using the created source intensity distribution and the defined patterning device.
The invention also provides a device manufactured in accordance with the above method of manufacturing a device.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate” and “target portion,” respectively.
In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g., having a wavelength in the range 5–20 nm), as well as particle beams, such as ion beams or electron beams.